Deep photo-lithography characterization of SU-8 resist layers

SU-8 layers of different thickness that were formed by spinning and soft baking or a casting process with a solvent content of not more than 4% were used for experimental investigation of deep UV lithography using modified radiation of a mercury lamp. The specific absorbance of the SU-8 layers have been measured in dependence on the wavelength to calculate the power of the absorbed radiation as a function of depth in the resist layer. The resist layers which were formed on a photo-mask have been exposed with a variation of exposure dose to study the residual thickness of the resist. There are threshold exposures for the formation of insoluble SU-8 resist layer and for the disappearance of shrinkage of the resist layer, which depends on the size of the irradiated area, on microstructure topology and on the resist thickness for fixed parameters of pre- and post-exposure baking. It has been shown that exact filtration of a low-intensity band of exposure radiation at 334 nm allows to reduce strong diffraction distortion in the upper layer of the resist. Microstructures with an aspect ratio of 25 in a SU-8 resist layer of 1 mm thickness have been obtained using a 100 m thick SU-8 resist layer as a filter for the UV radiation and with optimal dose of exposure.